The present invention relates to a method for the preparation of an aqueous emulsion of an organopolysiloxane or, more particularly, to a method for the preparation of an aqueous, emulsion of an organopolysiloxane of the oil-in-water type suitable for use as a fabric-finishing agent having excellent stability with a very small average diameter of the organopolysiloxane droplets and capable of imparting excellent softness and very pleasant feeling of touch to the fabric material treated therewith.
It is a well-established technology in the fabric industry that fabric materials of various kinds of natural fibers as well as synthetic fibers are subjected to a treatment with a fabric-finishing agent in order to upgrade the quality of the product such as smoothness in touch, softness and the like. Among a variety of types of fabric-finishing agents heretofore used in the fabric industry, silicone-based fabric-finishing agents are widely used including those comprising a dimethylpolysiloxane as the principal ingredient, those comprising an amino-modified organopolysiloxane as the principal ingredient and the like. These silicone-based fabric-finishing agents are used usually as an aqueous emulsion of the oil-in-water type prepared by the emulsification of the organopolysiloxane utilizing a surface active agent as the emulsifying agent.
The process of fabric-finishing treatment by using such an emulsion-type fabric finishing agent is undertaken usually under very unfavorable conditions against stability of the emulsion. For example, an aqueous emulsion of silicone as prepared is usually diluted by 10 to 100 times with addition of water before application to fabric materials. Further, such a diluted aqueous emulsion of silicone is subjected to a strong mechanical shearing force such as agitation, pressurization by pumping, squeezing and the like, sometimes, at a high temperature or brought into contact with other chemical compounds which may have an adverse effect on the stability of the emulsion.
Once such an aqueous emulsion of silicone is destroyed under the above mentioned adverse conditions, the silicone constituent is separated from the aqueous medium and unevenly deposited on the surface of the fabric material treated with the fabric-finishing agent to cause serious troubles such as a seemingly uneven color of the fabric material dyed in the preceding or concurrent dyeing procedure and a phenomenon of so-called oil spots which means that the spotwise areas on which the silicone is heavily deposited are seemingly in a deeper color than the other areas due to the difference in the refractive index.
Accordingly, extensive studies have been undertaken in order to improve the stability of an aqueous emulsion of silicones. One of the measures to improve the stability of an aqueous emulsion is to decrease the diameter of the silicone droplets dispersed in the aqueous medium as far as possible and proposals along this line have been made hitherto for a so-called "microemulsion", which is a seemingly clear liquid but microscopically not a uniform solution, and a method for the preparation thereof.
For example, U.S. Pat. No. 4,146,499 teaches a method for obtaining a clear microemulsion of an organopolysiloxane in a four-step process by utilizing two kinds of surface active agents having different solubilities in the organopolysiloxane. Further, U.S. Pat. No. 3,975,294 and U.S. Pat. No. 4,052,331 propose a method for the emulsification of a silicone by utilizing a combination of a specific emulsifying agent, organic acid and amine compound. These prior art methods, however, are not without problems as a practical method because the preparation process thereof is very complicated or an extremely large amount of an emulsifying agent is required so that the excellent fabric-finishing characteristics inherent in silicones are lost so greatly as not to fully exhibit the effect of imparting softness to the finished fabric material.
Alternatively, Japanese Patent Kokai No. 60-127327 proposes a method for obtaining an aqueous emulsion of a silicone having a diameter of the silicone droplets not exceeding 0.3 .mu.m by undertaking phase inversion in which an organopolysiloxane having polar groups is admixed with an emulsifying agent immiscible therewith and water to form a translucent oil concentrate which is then rapidly dispersed in water. This method, however, is defective because complicated process control is essential with utmost care in the control of the length of time and temperature in each step of the process if not to mention the insufficient fabric-finishing effect of the emulsion obtained by the method not to impart a good feeling of touch to the fabric material treated therewith.
Further alternatively, Japanese Patent Kokai No. 64-160 teaches a method in which a base emulsion of an organopolysiloxane having trifunctional siloxane units is gradually added dropwise to an aqueous solution containing a polymerization initiator for the emulsion polymerization and Japanese Patent Kokai No. 63-270875 proposes a method in which an aqueous emulsion of a carboxyl group-containing organopolysiloxane is neutralized. Neither of these methods, however, can impart the fabric material with a quite satisfactory feeling of touch.
Further, it is known that fabric materials can be imparted with a relatively satisfactory feeling of touch when the material is finished by using an aqueous emulsion obtained by the reaction of an organic acid added to an emulsion of an amino group-containing organopolysiloxane but such an emulsion is also not advantageous from the practical standpoint because the manufacturing process thereof is somewhat complicated involving a troublesome step of reaction.